I had a similar thing happen to me with my son's raptor. The exhaust valve got bent for no apparent reason during a start up. The mods to upgrade it wasn't worth the cost in my book. A new CDI, full exhaust, New carb slide (from a 80) and re jetting is going to run you $400 to $500 and that's after you repair your head. The added cost plus the worth of the 50 equaled the cost of a 90 which your boy will get much more life out of. I recommend just fixing your head and then selling it and upgrade to a 90. The size difference between the 2 isn't that much but the power and fun is.